The present invention relates, in general, to surge protection devices and, more specifically to secondary protection devices for protecting circuitry operating with low level voltages.
Bipolar circuitry may be exposed to damaging surges from the outside environment. Typically, the circuitry may be protected with a primary protector and a secondary protector. The primary protector is generally used to protect circuits operating with high level voltages, and the secondary protector is used to protect circuits operating with low level voltages, such as TTL logic level voltages.
Secondary protection devices must be fast acting to protect circuits against surges due to lightning or cross voltage appearing at the input terminals to the circuits. Some circuits, such as telephone communication (T1/E1) circuits, are also exposed to potentially destructive DC bias voltages and need isolation from the DC bias to prevent damage.
As new communication systems are introduced, secondary protection is provided by designing a protector circuit custom made for the system to be protected. Typically, the protector circuit may be part of other functional circuits in the system. Because the protector circuit is typically placed on the same chip as other functional circuits, precious space on the chip is allocated to the protector circuit. This approach is expensive and time consuming.
A need exists, therefore, for a secondary protection device that may be separate from the circuits to be protected. A need also exists for a secondary protection device that may be used for protecting various low level voltage circuits without having to custom design the protector circuit for each low level voltage circuit. Furthermore, a need exists for a protection device that may protect a circuit quickly and reliably.
To meet this and other needs, and in view of its purposes, the present invention provides a secondary protection device for protecting a circuit, including a first positive temperature coefficient (PTC) resistor, a bidirectional zener diode and a second PTC resistor connected in series across input terminals. The output terminals are connected across the bidirectional zener diode for providing a protected output to the circuit. A second zener diode is coupled between one end of the bidirectional zener diode and a ground potential for limiting an output voltage to the circuit. The diode power capabilities are based upon the protection requirements of the circuit. The PTC resistors each have a value of 4-6 ohms nominally. The secondary protection device is modularized and individually inserted in a punch down, 110-type connecting block for protecting the circuit.
In another embodiment, the secondary protection device includes a first positive temperature coefficient (PTC) resistor, a fast switching diode bridge and a second PTC resistor connected in series across input terminals. A first zener diode is connected between first and second nodes of the fast switching diode bridge. Second and third zener diodes are each connected between the first and second nodes respectively and a ground potential. Output terminals are connected between a third node of the diode bridge and the ground potential for providing a protected output to the circuit.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.